ARM: OMAP: fix return value check in realtime_counter_init()

In case of error, the function clk_get() returns ERR_PTR()
and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch)
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/timer.c

@@ -378,7 +378,7 @@ static void __init realtime_counter_init(void)
 		return;
 	}
 	sys_clk = clk_get(NULL, "sys_clkin_ck");
-	if (!sys_clk) {
+	if (IS_ERR(sys_clk)) {
 		pr_err("%s: failed to get system clock handle\n", __func__);
 		iounmap(base);
 		return;